Conventional MEMs mirrors for use in optical switches, such as the one disclosed in U.S. Pat. No. 6,535,319 issued Mar. 18, 2003 to Buzzetta et al, redirect beams of light to one of a plurality of output ports, and include an electro-statically controlled mirror pivotable about a single axis. Tilting MEMs mirrors, such as the ones disclosed in U.S. Pat. No. 6,491,404 issued Dec. 10, 2002 in the name of Edward Hill, and U.S. Pat. No. 6,677,695 issued Jan. 13, 2004 in the name of Dhuler et al, which are incorporated herein by reference, comprise a mirror pivotable about a central longitudinal axis. The MEMs mirror device 1, disclosed in the aforementioned Hill patent, is illustrated in FIG. 1, and includes a rectangular planar surface 2 pivotally mounted by torsional hinges 4 and 5 to anchor posts 7 and 8, respectively, above a substrate 9. The torsional hinges may take the form of serpentine hinges, which are disclosed in U.S. Pat. No. 6,327,855 issued Dec. 11, 2001 in the name of Hill et al, and in United States Patent Publication No. 2002/0126455 published Sep. 12, 2002 in the name of Robert Wood, which are incorporated herein by reference. In order to position conventional MEMs mirror devices in close proximity, i.e. with a high fill factor (fill factor=width/pitch), they must be positioned with their axes of rotation parallel to each other. Unfortunately, this mirror construction restraint greatly restricts other design choices that have to be made in building the overall switch.
When using a conventional MEMs arrangement, the mirror 1 positioned on the planar surface 2 can be rotated through positive and negative angles, e.g. ±2°, by attracting one side 10a or the other side 10b of the planar surface 2 towards the substrate 9. Unfortunately, when the device is switched between ports at the extremes of the devices rotational path, the intermediate ports receive light for fractions of a millisecond as the mirror 1 sweeps the optical beam past these ports, thereby causing undesirable optical transient or dynamic cross-talk.
One solution to the problem of dynamic cross-talk is to initially or simultaneously rotate the mirror about a second axis, thereby avoiding the intermediate ports. An example of a MEMs mirror device pivotable about two axes is illustrated in FIG. 2, and includes a mirror platform 11 pivotally mounted by a first pair of torsion springs 12 and 13 to an external gimbal ring 14, which is in turn pivotally mounted to a substrate 16 by a second pair of torsion springs 17 and 18. Examples of external gimbal devices are disclosed in U.S. Pat. No. 6,529,652 issued Mar. 4, 2003 to Brenner, and U.S. Pat. No. 6,454,421 issued Sep. 24, 2002 to Yu et al. Unfortunately, an external gimbal ring greatly limits the number of mirrors that can be arranged in a given area and the relative proximity thereof, i.e. the fill factor. Moreover, an external gimbal ring may cause unwanted reflections from light reflecting off the support structures, e.g. the torsion springs 12, 13 and the gimbal ring 14.
Another proposed solution to the problem, is disclosed in U.S. Pat. No. 6,533,947 issued Mar. 18, 2003 to Nasiri et al, which include hinges beneath the mirror platform. Unfortunately, these types of mirror devices include four separate pivoting levers requiring a great deal of space and costly multi-step fabrication processes. Consequently, the entire hinge structure can not be hidden beneath the mirror platform.
The solution to overcome the shortcomings of the prior art proposed by the inventors of the parent application listed above is to provide a high fill factor MEMs mirror device that can pivot about the same axis as an adjacent mirror. In a preferred embodiment the MEMs mirror device is relatively easy to fabricate, with an internal gimbal ring and applicable in high fill factor applications.
Typically in MEMs mirror devices the hinge and the reflective mirror are defined in the same semiconductor, e.g. Si, layer, whereby the minimum thickness requirement of the mirrors, currently 15 microns to prevent radius of curvature creep due to gold stress relaxation, dictates that the hinge width be <1.5 microns in order to achieve the desired spring constant. These hinge dimensions are at the limit of current DRIE processes both for feature resolution and wafer uniformity. By independently optimizing the mirror and hinge thicknesses, the required hinge width can be increased to within comfortable manufacturing tolerances without sacrificing mirror flatness. Moreover, voltage-tilt angle uniformity and yield will improve across the wafer.
An object of the present invention is to overcome the shortcomings of the prior art by providing a MEMs device, which includes a hinge structure hidden beneath the mirror platform, thereby decoupling the hinge thickness and the mirror thickness enabling the mirror curvature and the hinge dimensions to be optimized, while maintaining a high fill factor.